Over the past decade research from several laboratories has revealed two types of on-center retinal ganglion cells in the cat, X and Y cells. The general purpose of the research program is to assess the spatial and temporal characteristics of the excitatory and inhibitory mechanisms which underlie the responses of these two types of cells. Reseach from our laboratory in the past year has been concerned mainly with defining the spatial distribution of the inhibitory mechanisms that control the behavior of X cells. Earlier work from our laboratory (Hickey, Winters and Pollack, Vision Research 1973) suggested that lateral inhibition did extend through the center of the receptive field in X cells. Several studies conducted in the past year served as tests of that idea.